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If you are experiencing audio quality issues after updating to iTunes 9, please try this:
1. Open iTunes Preferences > Playback.2. Disable Crossfade songs, Sound Enhancer and Sound Check.3. Click OK.4. Open the Equalizer (, in the View menu on Windows).5. Disable the EQ by de-selecting the checkbox in the upper left.6. Play an affected track.
Two other things
http://support.apple.com/kb/TS1362 this link shows some settings to check in Xp
Also have you recently updated your audio driver for your computer?